Subguns
There are plenty of times when an assault rifle simply won't do. The miniature assault rifles, like the AR variants with 10" barrels and the like, spit hellfire out of the muzzle due to the short barrel. Also, a 5.56mm 55gr. ball round bullet will go right through a badguy. I've seen one go through an old steel pot helmet without slowing down. Overpenetration can be bad in an urban environment. Which makes me a little dubious about new subguns like the FN P90, and the HK PDW. Those smal bore high velocity rounds will go through an unarmored badguy, and to boot, lack the basic muzzle energy of even 9x19mm. The more muzzle energy, the more damage your bullet will do to your badguy.
As for russian subguns, head over to
http://www.izhmash.ru/first_en.html
You can see Izhmash line of Bizon-2 submachine guns. They have a groovy magazine like the Calico, and come in several different calibers. The 9x18mm version holds 64 rounds. There's also a 9x19 version, a 7.62x25mm version, a 9x17mm version, and a 9x18mm version with an integral suppressor.
Some other interesting subgun info. Most people don't know that the IMI Uzi comes in three flavors. Full size, with the stock that folds up under the rear of the weapon and with a fire rate of about 600rpm, mini, with a side folding stock and a fire rate of about 1000rpm, and the micro pistol sized version, which is smaller than a MAC-10 and has no stock, and has a rate of fire of about 1200rpm.